New U-Pb SHRIMP detrital zircon ages from the Ambato and La Cébila metamorphic complexes (Eastern Sierras Pampeanas, Argentina). Evidence for the final docking of the Rio de la Plata craton

SEBASTIÁN OSVALDO VERDECCHIA; EDGARDO GASPAR BALDO; CESAR CASQUET; CARLOS WASHINGTON RAPELA; ROBERT JOHN PANKHURST; MARK FANNING

Brasilia

Simposio; VII South American Symposium on Isotope Geology; 2010

Universidad de Brasilia, Brasil

Knowledge of the Early Paleozoic stratigraphy of the Eastern Sierras Pampeanas is hindered by metamorphism and deformation resulting from the Pampean (545-515 Ma) and Famatinian (490-465 Ma) orogenies. U-Pb SHRIMP dating of detrital zircons is a useful tool to decipher stratigraphy and to infer paleogeographic scenarios. Pre-Pampean sedimentary protoliths are characterized by an almost bimodal population of zircons (1250-960Ma and 680-570Ma) with a minor contribution of ca. 1.9 Ga grains. Post-Pampean metasedimentary rocks however show a remarkable new peak at 2.0-2.1 Ga. This evidence led Schwartz & Gromet (2004) and Rapela et al. (2007) to propose that the nearby Rio de La Plata Craton (ca. 2.1 Ga) was not a source area for the pre-Pampean sediments and that its position was attained during or after the Pampean orogeny. At Qda de La Cébila (ca. 28º45’S-66º30’W) two metasedimentary successions are found: the La Cébila (LCMC) and Ambato (AMC) metamorphic complexes. One sample from each succession was run for U-Pb SHRIMP dating of detrital zircons. Both contain Pampean zircons (AMC peak at 520 Ma; LCMC peak at 512 Ma) that provide a maximum age for the sedimentary protoliths. The AMC sample also shows peaks of Brasiliano (n=9), Grenvillian (n=9) and minor Neoarchean age. The LCMC sample however shows a complementary well defined peak of Paleoproterozoic (Rio de la Plata craton, n=5) age. These data are interpreted as evidence for the AMC being older than the LCMC. On the other hand we infer that the present position of the Rio de la Plata craton was reached between the deposition of the two sedimentary successions, after the Pampean Orogeny, i.e., in the Upper Cambrian.